1. Field of the Invention
This invention relates to medical imaging and more particularly relates to a system and methods for active suppression of superior tagging in flow-sensitive alternating inversion recovery.
2. Description of the Related Art
The perfusion of the cerebral tissue bed is a very important physiological index for evaluating function and viability. One perfusion imaging technique is pulsed arterial spin labeling (PASL). One of the most widely used PASL techniques is the flow-sensitive alternating inversion recovery (FAIR) technique.
FIG. 11 describes a schematic diagram for ASL imaging according to the prior art. The labeling of arterial blood is proximal to the tissue of interest, as shown by the blue plane on the left portion of the figure. After a delay to let labeled blood to arrive at the tissue sites, imaging acquisitions may be performed. In some ASL techniques, such as EPISTAR, control experiments are typically done using the symmetric labeling RF pulse at the distal site to minimize the MT effects (light blue plane at right).
FAIR is a symmetric PASL technique, and originally used selective inversion and non-selective inversion for control and labeling imaging. The selective and non-selective inversion RF pulses were typically hyperbolic secant pulses with duration about 15 ms. To avoid slice profile imperfections at the edges or minimize substantial subtraction errors due to the transition regions of the inversion slabs, the selective inversion slab is made wider than the imaging slab. The non-selective inversion is achieved by using the same hyperbolic secant pulse without the slab selective gradient. In those earlier studies, transmit/receive head coils were dominantly used. However, the use of transmit/receive head coil for ASL perfusion studies may give lower labeling efficiency at the edge of the head coil. Due to the limited size of the transmit/receive head coil, to have enough arterial blood labeled, a larger labeling slab has to be used, making ASL perfusion studies mainly restricted to the superior part of the brain.
There are still some unsolved problems with FAIR and similar sequences. First, the intrinsic two-sided labeling generates significant venous artifacts due to inflowing spins from the superior inversion slab in perfusion studies using limited coverage, especially when such studies are focused on the mid-brain and inferior parts of the brain containing large veins, such as the occipital lobe with sagittal sinus and the cerebellum with transverse and straight sinuses. Second, the two-sided labeling of FAIR can introduce additional potential confounds for CBF quantification using single subtraction methods when limited coverage imaging slabs are used and imaging slice(s) contain curved or tortuous arteries that go through the imaging slice(s) and come back. For accurate CBF quantification, this superior inflow should be included in the model. More simplistically, if the single subtraction method and single blood compartment model are used, the temporal width for the superior bolus should also be defined, as usually done for the inferior bolus with QUIPSS II or Q2TIPS. However, this may affect the bolus defined in the inferior side of the imaging slab, making CBF quantification using FAIR in this circumstance problematic.
With parallel imaging, multiple channel phased array coils are used for signal reception, while the body coil is typically used as the transmit coil. But, non-selective inversion with the body coil produces extra problems for FAIR. One problem is that the repetition time has to be long enough to allow the complete relaxation of labeled spins before the next non-selective inversion. Otherwise, the labeling efficiency or the signal-to-noise ratio of the perfusion map will be lower. Moreover, the CBF quantification model assumes that in the labeling experiment of the FAIR technique, the blood outside of the imaging slab should be fully relaxed. An even worse problem for body coil transmission is that the very long bolus duration from the non-selective inversion will present labeled spins in larger arteries of the imaged slice(s) during signal acquisition, generating spurious hyper-intense signals in both perfusion-weighted images and CBF maps and resulting in the overestimation of cerebral blood flow.
The referenced shortcomings are not intended to be exhaustive, but rather are among many that tend to impair the effectiveness of previously known techniques in disease diagnosis and management; however, those mentioned here are sufficient to demonstrate that the methodologies appearing in the art have not been satisfactory and that a significant need exists for the techniques described and claimed in this disclosure.